iOS 11.2.2 includes security improvements to Safari and WebKit to mitigate the effects of Spectre (CVE-2017-5753 and CVE-2017-5715).

Apple says it “recommends” the update “for all users.”

To download the update, go to Settings > General > Software Update (the preferred way, and this way the patch is only about 75MB), or connect your iPhone to a computer running iTunes and then carry out the update (this is slower and downloads the entire iOS 11.2.2 package, which is several gigabytes, as opposed to just the smaller update).